1. Field of the Invention
The present invention relates to a copper-clad laminate and a printed wiring board. More particularly, the present invention relates to a printed wiring board having a fine-pitch wiring pattern and exhibiting a high etching factor, and further relates to a copper-clad laminate which can be suitably employed in the production of such a printed wiring board.
2. Prior Art
The copper-clad laminate used in the production of a printed wiring board is usually obtained by bonding a copper foil as a conductive layer to one side or both sides of a substrate made from a glass-reinforced epoxy resin material or the like. A wiring pattern is formed by etching the copper foil. The copper foil and the substrate are bonded together under pressure while heating with or without an adhesive interposed therebetween.
Electrolytically produced copper foil is generally employed. The electrolytic copper foil is obtained by electroplating a copper foil on a metal drum. One side (the glossy or shiny surface) of the electrolytic copper foil is relatively smooth, while the other side (the matte surface) is generally rough. When viewed in cross-section, the texture on the glossy surface, which is formed next to the drum during the initial stage of electrodeposition, exhibits random orientation of small crystal grains. As electrodeposition proceeds away from the drum toward the outer or matte surface, the crystal grains become oriented in the direction of the electrodeposition and become larger.
In the conventional electrolytic copper-clad laminates, the electrolytic copper foil is bonded on its matte surface side to the substrate for the reasons of greater bonding strength and easier handling. The circuit pattern is formed by applying a resist pattern and then etching from the exposed glossy (shiny) surface down to the substrate. In order to improve the peel strength from the substrate, copper particles of 0.2 to 3.5 .mu.m diameter are applied to the matte surface of the copper foil.
Although the circuit lines should have vertical sides defined by the resist pattern, formation of a wiring pattern by etching the above-mentioned copper-clad laminate produces circuit lines having sides which are not perfectly vertical, due to the effects of crystal orientation and crystal grain boundaries on etching. Sharp edges of the conductor pattern cannot be obtained. The accuracy of the circuit lines may be defined by the "etching factor", as is shown in FIG. 1. When the etching factor is small, the top of a circuit line is narrow and the bottom is broad due to horizontal etching in addition to the desired vertical etching. Thus, the spacing (gap) between the conductor lines in a circuit pattern is reduced, thereby causing a problem that migration is likely to occur. Thus, since the circuit lines are not perfectly rectangular, formation of fine patterns is difficult. In commercial practice, the etching factor should be as large as possible, preferably above about 4.
Decreasing the thickness of the copper foil is one means of obtaining a fine pattern. In this case, the cross-sectional area of the conductor becomes so small that the quantity of available electric current is small. Accordingly, to allow ample current passage, it is necessary to increase the cross-sectional area of the conductor by copper plating the wiring pattern obtained by the etching. Also, when the copper layer is thin, lead bending occurs in a film carrier having an inner lead, e.g., TAB.
Moreover, in the etching of the above-mentioned conventional copper-clad laminate for fine pattern formation, electrodeposited copper particles remain adherent to the substrate of the printed wiring board at the bottom of the conductor pattern formed by etching, and the remaining electrodeposited copper particles cause the insulating resistance between the conductors to decrease, in extreme cases causing short-circuiting.
Various reports have been published describing the formation of copper electrodeposits on the copper foil. For example, the electrodeposition of a layer of copper particles onto both sides of a copper foil has been reported. However, this copper foil is used as an inner conductor layer in a multilayer printed wiring board and the purpose of the electrodeposition is to provide a substitute for the black oxide treatment aiming at ensuring the adhesion of the inner copper foil to the substrate at the secondary bonding. A pattern printing is applied to the glossy surface side.
In another example, Japanese Patent Application Laid-Open Gazette No. 29740/1993 describes an electrolytic copper foil having both sides thereof provided with gloss plating, with one of the sides of the copper foil having been subjected to surface treatment, such as roughening. In this example also, the etching is performed from the glossy surface side.
The lamination of the glossy side of an electrolytic copper foil to a substrate has been disclosed, but the effect of this on the etching process has not been recognized. For example, Adler in U.S. Pat. No. 4,997,516 discusses the need to deposit a roughening layer on the shiny ("glossy") side of the foil in order to provide adhesion to the substrate. However, he notes that this is not commonly done. Adler further comments on the difficulties experienced with foils which have roughening deposits on both the shiny and matte sides. In WO 94/21097, Polyclad Laminates advocates bonding the shiny side of a foil to the substrate in order to obtain advantages in the manufacture of multi-layer circuit boards. Neither of these disclosures suggests that improved etching could result, perhaps because neither recognized that the degree of roughness on the matte side is an important factor in achieving circuit lines having a high etching factor.
As suggested above, the copper grain size at and near the glossy surface is small and the grains are randomly shaped. Consequently, acid used for etching circuit lines can travel along the grains boundaries horizontally as well as in the desired vertical direction. As result the top of the circuit lines generally will be narrower than the bottom when etching is carried out from the glossy side. The copper grains at the matte surface are generally columnar or planar in shape and extend vertically into the copper foil. Therefore, etching from the matte side minimizes the horizontal etching ordinarily observed when etching from the glossy side and improves the etching factor. However, the degree of roughness on the matte side of the foil is an important factor and should be within certain limits, as will be seen from the description of the invention below.